The present invention relates to a surface acoustic wave device having interdigital electrodes and, more particularly, to a surface acoustic wave device which is formed on a piezoelectric substrate and operates with a VHF (Very High Frequency) or an UHF (Ultra High Frequency).
In a conventional resonator type surface acoustic wave device, the transverse mode (fundamental mode), in which the energy is distributed in a direction perpendicular to the propagating direction of a surface acoustic wave, becomes the major propagating mode, while high-order modes sometimes considered as spurious.
In order to suppress such the spurious, as a conventional technique, Japanese Patent Laid-Open No. 62-219709 discloses a surface acoustic wave resonator in which, regarding the shape of the interdigital electrodes, the ratio of the overlap width of the electrode fingers to the aperture width of the electrode fingers is 65% to 75% and this overlap width is constant within the interdigital electrodes.
In this surface acoustic wave resonator, as shown in FIG. 8, the potential present in the +/- range of the high-order transverse mode is effectively canceled by setting a ratio W/W0 of an overlap width W to an aperture width W0 of interdigital electrodes 71 and 72 from 65% to 75%, thereby suppressing the higher-order transverse mode. The characteristic feature of this conventional technique resides in that the ratios of the overlap widths to the aperture widths of the interdigital electrodes which are present in the surface acoustic wave device are equal among all the interdigital electrodes.
In the conventional technique described above, however, when a plurality of high-order modes are present, the device characteristics are degraded by the spurious in the mode on an order different from the order of the mode which is to be suppressed.
The reason for this is as follows. As shown in FIG. 8, a surface acoustic wave device has one or more interdigital electrodes, and the ratio W/W0 of the overlap width of its interdigital electrodes is constant throughout the surface acoustic wave device. In a case wherein only one high-order transverse mode is present, this high-order transverse mode can be suppressed.
When a plurality of high-order transverse modes are present, as in a case wherein the aperture width W0 is increased or the thickness of the metal thin film that forms the interdigital electrodes is increased, all the high-order transverse modes cannot be suppressed simultaneously.